Lord Dannatt told Sky News: "The old dictum that my enemy's enemy is my friend just might have some credence in this less than satisfactory and pretty extraordinary set of times that we are in."

But Britain has ruled out negotiating with Mr Assad and there has been no sign of any shift in US policy towards Syria, where the civil war has killed at least 191,000 people.

On Sunday, IS fighters captured the Tabqa air base in northeast Syria after days of fighting that cost more than 500 lives, according to a monitoring group.

Video:Hammond Warns Of Extremist Threat

"Islamic State has no borders and the faster we move against it, the more we diminish its danger," Mr Moualem said.

Meanwhile in Iraq, Kurdish peshmerga forces have retaken three villages in the Jalawla area northeast of Baghdad from jihadists and also held off two assaults on the Shia Turkmen-majority town of Tuz Khurmatu, officials said.

In Baghdad, a suicide bomber blew up his explosives-laden vest among Shia worshippers who were leaving a mosque after noon prayers, killing at least 12 people.